This invention relates to a method for preparing wafers from a single crystal of an oxide piezoelectric material, more specifically to a method for cutting X-cut wafers from such single crystal.
The X-cut wafers obtained by cutting the single crystal of oxide piezoelectric material in a direction perpendicular to the X-axis are available as substrates of surface wave intermediate frequency filters for color television receivers, for example. For the convenience of cutting the X-cut wafers from the single crystal, such single crystal is generally produced and used in the form of a cylinder as-grown in the X-axis direction, that is, extending with the X-axis as the longitudinal direction. Such cylindrical single crystal may be produced by the pulling-up method, such as Czochralski method, or pulling-down method.
Now I will describe a typical example of conventional method for preparing X-cut wafers from the cylindrical single crystal of oxide piezoelectric material. (1) First, the Z-axis direction of the single crystal is determined by the X-ray Raue method or the like, and the single crystal is cut at the opposite curved sides by means of a diamond wheel to provide two planes perpendicular to the Z-axis (FIG. 1A). Such cutting is called Z-axis cutting. The length of the edge where an X-plane 2 of a single crystal 1 and each formed Z-plane 3 cross each other is required to be larger than the radius of the original cylinder. Poling is performed by applying a voltage of 5 to 10 V/cm between both Z-planes. Poling changes the as-grown multi-domain single crystal into a single-domain single crystal. (2) Secondly, the side portion is cut by means of a diamond wheel by a plane in parallel with +112.2.degree. Y-direction determined by the X-ray Raue method or the like to form an orientation flat 4 (FIG. 1B). (3) Finally, the single crystal is cut by means of a diamong wheel in a direction perpendicular to the X-axis and thus an X-cut wafer 5 with the orientation flat 4 is obtained.
According to the conventional method, the single crystals have frequently been cracked by Z-axis cutting, cutting for the formation of orientation flat, and/or cutting for obtaining wafers. In particular, cracks have been caused very often when large-diameter single crystals are cut along the Z-axis. The reason for this is that cylindrical as-grown single crystals involve accumulated distortion therein, and it is generally known that cracks are liable to be caused on a plane &lt;102&gt; upon which distortion is centered. Such outbreak of cracks in cutting would reduce substantially the yield rate of wafers, thereby prominently raising the production cost of resultant wafers.
Further, the surface area of the X-cut wafer prepared by the above-mentioned conventional method was revealed to be only equivalent to approximately 85% of that of the X-plane of the cylindrical as-grown single crystal, which meant nothing but want of economy.
An object of this invention is to provide a method for preparing wafers from a rhombohedral single crystal of an oxide piezoelectric material at a high yield rate as well as with high efficiency.
Another object of the invention is to prevent cracks from being caused when cutting the cylindrical single crystal.